1. Field of the Invention
This invention relates to a hybrid device and a memory apparatus realized by using such hybrid devices. It also relates to an information reading method.
2. Related Background Art
Various devices utilizing the magnetoresistance effect such as thin film magnetic heads have been developed in recent years. In the course of developing such devices, magnetic thin film memories using magnetoresistance elements that can replace currently popular DRAMs and EEPROMs have been proposed. Since the electric resistance of a magnetoresistance element varies remarkably depending on its state of magnetization, non-volatile solid-state memories can be realized by combining such elements and semiconductor elements in a manner referred to as hybridization.
For instance, Japanese Patent Application Laid-Open No. 6-84347 describes a magnetic thin film memory comprising memory cells that are formed by combining magnetoresistance elements and switching elements such as field effect transistors (hereinafter referred to as FETs). FIG. 1 of the accompanying drawings schematically illustrates the circuit configuration of a magnetic thin film memory proposed in the above cited patent document.
FIG. 1 is a schematic circuit diagram of a known memory apparatus realized by using known memory devices (hybrid devices of magnetic thin film elements/semiconductor elements). FIGS. 2A and 2B illustrate two different arrangements of magnetic thin films in a memory device as shown in FIG. 1. FIGS. 3A and 3B illustrate two different states of magnetization of a magnetoresistance element as shown in FIG. 1.
Referring to FIG. 1, the memory apparatus comprises a plurality of memory cells, each having a magnetoresistance element 1 for storing information as a function of the state of magnetization of the magnetic material of the element and a switching element 2 for recording/reproducing information. Typically, FETs are used for the switching elements 2 and a write line 5 is connected to the source electrode of each of the FETs. Each of the magnetoresistance elements 1 is connected at an end thereof directly to the write line 5 of the device and the other end to the ground potential.
The memory apparatus additionally comprises a plurality of selection lines 4 for applying a voltage to each of the switching elements 2 to turn it on or off and a plurality of data lines 3 for writing data to or reading data from each of the memory cells, said selection lines 4 and said data lines 3 being arranged to form a grid. Memory cells are arranged at the respective intersections of the selection lines 4 and the data lines 3. The gate electrodes of the FETs of the memory cells of each row are commonly connected to the related selection line 4, whereas the drain electrodes of the FETs of the memory cells of each column are commonly connected to the related data line 3. A resistor 6 is connected to each of the data lines 3 so that information is recorded in each of the memory cells by applying a predetermined voltage to the related data line 3 by way of the related resistor 6.
In FIG. 1, the magnetoresistance elements 1 are discriminated from each other by means of reference symbols 1aa through 1ac and 1ba through 1bc, whereas the switching elements 2 are discriminated from each other by means of reference symbols 2aa through 2ac and 2ba through 2bc. Similarly, the write lines 5 are identified by respective reference symbols 5aa through 5ac and 5ba through 5bc, whereas the data lines 3 are identified by respective reference symbols 3a through 3c and the selection lines 4 are discriminated from each other by means of reference symbols 4a and 4b.
As seen from FIGS. 2A and 2B, a magnetoresistance element 1 is formed from a giant magnetoresistance (GMR) film obtained by laying a magnetic layer 20 showing a large coercive force and a magnetic layer 21 showing a small coercive force repeatedly for several times with a non-magnetic layer 22 interposed therebetween to produce a multilayer structure. A GMR film has a remarkable property that it shows a small electric resistance when both the magnetic layers showing a large coercive force and the magnetic layers showing a small coercive force are magnetized in a same direction whereas it shows a large electric resistance when the magnetic layers showing a large coercive force and the magnetic layers showing a small coercive force are magnetized in opposite directions.
Now, a method of recording information to and reproducing information from a memory apparatus as shown in FIG. 1 will be discussed below.
When storing "1" in the magnetoresistance element 1ac, a voltage of +V.sub.3 is typically applied to the data line 3c. When a voltage V.sub.4 is applied to the selection line 4a under this condition, the switching element 2ac is turned on and a relatively large electric current I.sub.1 flows to the magnetoresistance element lac and the write line 5ac in a direction running from the bottom surface to the top surface of FIG. 3A. Then, a magnetic field H.sub.1 is applied to the magnetoresistance element lac due to the electric current I.sub.1 so that consequently the direction of magnetization of the magnetic layer b operating for storing information and showing a small coercive force is made to agree with the direction of the magnetic field H.sub.1, or the leftward direction in FIG. 3A.
When, on the other hand, storing "0" in the magnetoresistance element 1ac, a voltage of -V.sub.3 is typically applied to the data line 3c. When a voltage V'.sub.4 is applied to the selection line 4a under this condition, the switching element 2ac is turned on and a relatively large electric current I.sub.0 flows to the magnetoresistance element lac and the write line 5ac in a direction running from the top surface to the bottom surface of FIG. 3B. Then, a magnetic field H.sub.0 is applied to the magnetoresistance element lac due to the electric current I.sub.0 so that consequently the direction of magnetization of the magnetic layer b operating for storing information and showing a small coercive force is made to agree with the direction of the magnetic field H.sub.0, or the rightward direction in FIG. 3B.
Since the switching element 2ac is turned on only when a predetermined voltage is applied to the related selection line 4a, no electric current will flow to the magnetoresistance element 1bc commonly connected to data line 3c with the magnetoresistance element 1ac. Additionally, since no electric current flows to the data lines 3 except the data line 3c to which a predetermined voltage is applied, no electric current will flow to the magnetoresistance elements 1aa, 1ab commonly connected to selection line 4a with the magnetoresistance element 1ac.
The magnetic layer 20 showing a large coercive force is normally magnetized in the rightward direction in FIGS. 2A and 2B and, therefore, the resistance of the magnetoresistance element 1ac is relatively large when "1" is stored in it but relatively small when "0" is stored in it.
When reproducing the information stored in the magnetoresistance element 1ac, a constant electric current I.sub.3 is made to flow through the data line 3c for data reproduction and an appropriate voltage is applied to the selection line 4a to turn on the switching element 2ac. Then, an electric current flows to the write line 5ac and the magnetoresistance element 1ac as a result. Therefore, a potential difference V.sub..alpha..beta. will be produced between point .alpha. (the drain voltage of the switching element 2ac) and point .beta. (ground potential).
As pointed out above, the electric resistance of the magnetoresistance element 1ac varies depending on the state of magnetization so that consequently the value of the potential difference V.sub..alpha..beta. also varies. Therefore, the state of magnetization of the magnetoresistance element 1ac and hence the information stored there can be known by observing the potential difference V.sub..alpha..beta..
Thus, in the above described known memory devices (hybrid devices of magnetic thin film elements/semiconductor elements), the information stored in a memory cell can be identified by detecting the voltage difference produced as a function of the change in the resistance of the magnetoresistance element connected to the source electrode or the drain electrode of the switching element (e.g., FET) of the memory cell.
However, since the voltage difference is proportional to the product I.multidot..DELTA.R of the drain current I and the difference of resistance .DELTA.R of the magnetoresistance element, it is difficult to obtain a voltage difference required for identifying the stored information if the difference of resistance .DELTA.R of the magnetoresistance element is small.
Additionally, with a magnetoresistance element showing a small difference of resistance .DELTA.R, while a large drain current will be required to produce a voltage difference necessary for identifying the stored information, a large drain current entails a problem of a large power consumption of the magnetic thin film memory device.
Still additionally, even a magnetoresistance element showing a relatively large difference of resistance .DELTA.R requires the use of a sense circuit for amplifying the difference of resistance because the potential difference produced by the difference of resistance is small. Then, there arises a problem of complex circuit configuration.
In an experiment, a memory cell as shown in FIG. 4 was used and a constant current source I.sub.1 was connected to the drain electrode of the FET. Then, the voltage V.sub.AB between the drain electrode (point A) and the ground potential (point B) was observed by varying the electric current being supplied to the drain electrode. The output voltage of constant voltage source V.sub.1 was invariably held to 5V.
FIGS. 5 and 6 show some of the obtained results. FIG. 5 is a graph showing the relationship between the output current I of the constant current source and the voltage V.sub.AB when a magnetoresistance element comprising a spin scattering film adapted to show two resistance values of 10 .OMEGA. and 11 .OMEGA. (with a ratio of change of resistance of 10%) and hence operates as binary element.
FIG. 6 is a graph showing the relationship between the output current I of the constant current source and the voltage V.sub.AB when a magnetoresistance element comprising a spin tunnelling film adapted to show two resistance values of 4.0 K.OMEGA. and 4.8K.OMEGA. (with a ratio of change of resistance of 20%) and hence also operates as binary element.
As seen from FIG. 5, with the magnetoresistance element comprising a spin scattering film, the voltage difference observed for the voltage V.sub.AB due to the difference of resistance of the magnetoresistance element was 3 mV when the output current I of the constant current source was 3 mA. It was difficult to reliably detect such a low voltage within a memory and hence an amplifier circuit was required to boost the detected voltage to a voltage level of several volts before outputting it to the outside of the memory.
With the magnetoresistance element comprising a spin tunnelling film as shown in FIG. 6, the voltage difference observed for the voltage V.sub.AB due to the difference of resistance of the magnetoresistance element was .DELTA.R.times.I=0.8 K.OMEGA..times.0.689 mA=551 mV when the output current I of the constant current source was 0.689 mA. Therefore a sense circuit was required to boost the detected voltage to a voltage level of several volts before outputting it to the outside of the memory.
When the output current I of the constant current source was raised to 0.690 mA, the voltage between the gate and the source was 5V-(0.690 mA.times.4.8 K.OMEGA.)=1.688V because of the rise in the source voltage. Since this value was equal to the threshold voltage of the FET, the latter was not turned on and no electric current was observed between the source and the drain. In other words, the output current I of the constant current source could not be raised above 0.690 mA. Then, it was impossible to reliably detect the information stored in the memory by raising the electric current flowing to the drain. This is because, in the arrangement of FIG. 1, the FETs are used only as switching elements and are not adapted to operate as amplifiers.
Additionally, when storing information in the above described memory device by generating a magnetic field by means of the write line arranged adjacent to the magnetoresistance film of a selected magnetoresistance element, the electric current made to flow for storing information in the element also flows to the magnetoresistance film because a same and identical current path is used both for observing the resistance of the magnetoresistance film and for recording information. Then, only an insufficient electric current flows through the write line to make it impossible to efficiently generate a magnetic field from the write line and record information on a reliable basis.